352 E. B. MATHEWS AND W. J. MILLER — COCKEYSVILLE MARBLE 



which themselves are highly crinkled, may var}' from to 90 degrees, 

 representing in their general structure open folding, but in their intimate 

 structure the folds are frequently much compressed and often overturned 

 and slightly faulted. 



IGNEO US ROCKS 



Through these several sedimentary formations have been intruded 

 igneous rocks ranging in composition from granites to peridotites. The 

 main development of the igneous rocks, however, lies outside of the field 

 selected, only the extreme members being represented, in the granite 

 between Warren and Cockeysville and in the serpentinized peridotite of 

 the Bare hills. These igneous rocks were intruded either during the 

 folding of the rocks or subsequently. 



There are also several lines of diabase boulders representing dikes 

 which have been correlated with the great Triassic intrusions farther 

 north. 



A real Distribution of the Rocks 



IN GENERAL 



The rocks of the region, as shown by the accompanying sketch map 

 (figure 1), may be roughly grouped into three broad belts extending 

 parallel to the strike of the formations from northeast to southwest. On 

 the south are two anticlinal areas of the Baltimore gneiss, separated from 

 each other along the strike by a fault and sharp folding, while on the 

 north is an oval anticlinal area of Baltimore gneiss about 15 miles in 

 length and 5 miles in breadth in its widest point. Between these two 

 areas in a broad synclinal trough occur in regular succession the Setters 

 quartzite, Cockeysville marble, and Wissahickon schist, the latter occu- 

 pying the larger portions of the area and connecting on the north, east, 

 and west with the larger body of Wissahickon, which extends diagonally 

 across Maryland from the southwestward continuation of the type Wissa- 

 hickon of Pennsylvania to the Potomac river on the west. 



DISTRIBUTION OF BALTIMORE GNEISS 



Southern area. — The southern development of the Baltimore gneiss on 

 the south side of the Greenspring and Mine Branch valleys falls into two 

 distinct areas, separated by the limestone and Wissahickon valley of lake 

 Roland and possibly by a fault. On the west is a lenticular anticlinal 

 mass entirely surrounded by the Setters Ridge quartzite, wherein the 

 bands of Baltimore gness stand at a high angle, ranging from 40 to 75 

 degrees at the center, and strike parallel to the major axis of the ellipse 

 except near the ends, where their strike follows the general contour of 



